Acyloxysilanes have found various uses in the chemical industry. They are suitable, for example, as crosslinking silicon compounds in the preparation of compositions which can be stored in the absence of water and can be hardened to elastomers at room temperature in the presence of moisture. Such compositions are obtained by mixing diorganopolysiloxanes containing end groups which can undergo condensation and crosslinking silicon compounds. Examples of acyloxysilanes which are suitable for this purpose are vinyl-, methyl- and ethyltriacetoxysilanes. Carboxylic acid chlorides are important and, in some cases, valuable raw materials for the synthesis of organic compounds.
It is known that alkanoyloxysilanes can be prepared continuously by reaction of organochlorosilanes with alkanoic acids at elevated temperature in a column (DE-PS 28 01 780). The alkanoic acid is passed in vaporized form from the bottom upwards in countercurrent with the organochlorosilane and reacts to form alkanoyloxysilane and hydrogen chloride.
A disadvantage of this procedure is that the hydrogen chloride formed is in contact with the alkanoic acids employed for relatively long periods of time under conditions under which the formation of water and alkanoyl chlorides takes place. This results in additional siloxane formation and requires expensive low-temperature cooling at the top of the column so that the substances entrained in the stream of hydrogen chloride according to their vapor pressure are deposited and the hydrogen chloride is rendered usable for purposes for which particular purity requirements are imposed. Another disadvantage of this procedure is that metering of the alkanoic acids introduced into the column must be regulated by an expensive device which is controlled by a temperature measurement point in the lower part of the column.
It is also a further disadvantage of the procedure of DE-PS 28 01 780 that the reaction, which is associated with the release of large amounts of gas, must be carried out in vacuo, which necessitates particular pressure stabilization equipment.
Because of the deficiencies associated with the procedure according to DE-PS 28 01 780, ethyltriacetoxysilane is obtained in the distillation column with an ethyltriacetoxysilane content of only 94%, coupled with a content of 2% of acetic acid and evidently 4% of siloxanes, as well as a content of hydrolyzable chlorine of about 50 ppm.
Only a slight improvement to this situation is achieved if in this known procedure the addition of the alkanoic acid together with the organochlorosilane is carried out in the lower part of the column and additional alkanoic acid is introduced in the upper part of the column (DE-PS 32 21 702).
It is also known that the reaction of organochlorosilanes can be carried out with anhydrides corresponding to the alkanoic acids; however, only a discontinuous procedure has been described for this reaction.
There was therefore the problem of discovering a continuous process, which can be used on a large industrial scale, for the preparation of acyloxysilanes which does not have the deficiencies of the procedures according to the prior art described herein, and in which a high-quality by-product in the form of carboxylic acid chloride is obtained instead of the relatively useless by-product hydrogen chloride.